Two-state negative feedback avalanche diode having a control element for determining load state

ABSTRACT

A negative feedback avalanche diode for detecting the receipt of a single photon is described. The photodetector comprises a load element having two load states, one characterized by high impedance and the other characterized by low impedance. The load state of the load element is controlled by a control signal generated within the negative feedback avalanche diode itself.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser.No. 61/577,227, filed 19 Dec. 2011, entitled “Two-state NegativeFeedback Avalanche Diode,” which is incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under W31P4Q10C0164awarded by the Defense Advanced Research Projects Agency (DARPA). TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to avalanche photodetectors in general,and, more particularly, to single-photon avalanche photodetectors.

BACKGROUND OF THE INVENTION

Photodetectors capable of detecting a single photon (i.e., a single“particle” of optical energy) are useful in many applications. To date,most of these applications have relied on the use of single-photondetectors such as photomultiplier tubes (PMTs) or single-photonavalanche detectors (SPADs) that are silicon-based, and are thereforecapable of efficiently detecting only photons that have a wavelengthwithin the range of approximately 250 nanometers (nm) to approximately900 nm. New applications are emerging, however, that requiresingle-photon detectors that can operate at high speed (>1 MHz) and atlonger wavelengths (>1000 nm). Such devices would find use in areas suchas: quantum information processing, quantum computing, quantumcryptography, and quantum teleportation and communications;low-light-level imaging and other high-performance imaging applications;and others. Unfortunately, currently available SPADs do not have thecombination of high operational speed and wavelength range required formany of these applications.

An avalanche photodiode (APD) is a type of photodetector that is capableof providing extremely high sensitivity. An APD derives its name fromthe manner in which its output signal is created. When an APD absorbsphotons, their energy excites electrons normally bound in the atomiclattice of the APD material to create free electrons. Each freedelectron leaves behind a positively charged vacancy (i.e., a “hole”) inthe crystal structure. These electrons and holes are free-chargecarriers that can flow freely through the structure of the APD.

In the presence of an electric field (due to a bias voltage appliedacross the APD), these free-charge carriers are accelerated through aregion of the avalanche photodiode known as the “multiplication region.”As the free-charge carriers travel through the multiplication region,they collide with other electrons and holes bound in the atomic lattice,thereby generating more free-charge carriers through a process called“impact ionization.” These new free-charge carriers also becomeaccelerated by the applied electric field and generate yet morefree-charge carriers.

When operated in “Geiger mode,” an APD can be made sensitive enough todetect even a single photon, and a device designed specifically for thispurpose is referred to as a single-photon avalanche diode (SPAD). InGeiger-mode operation, a SPAD is “armed” by biasing it with a voltagethat is above its breakdown voltage, which is the voltage bias levelabove which free-charge carrier generation can become self-sustainingand result in a run-away avalanche. Arming a SPAD puts it in ameta-stable state in which absorption of a single photon can give riseto a runaway avalanche that results in an easily detectable macroscopiccurrent. This avalanche event can occur very rapidly and efficiently andit is possible to generate several hundred million free-carriers from asingle absorbed photon in less than one nanosecond (ns).

In order to prepare the SPAD for re-arming once this current isgenerated, the avalanche current must be halted. This is done with aprocess referred to as “quenching,” wherein the bias voltage is reducedto a value sufficiently close to the breakdown voltage that theavalanche can spontaneously terminate.

Controlling voltage bias to arm and quench an APD is one of the primarychallenges for Geiger-mode operation and the rate at which asingle-photon detector can be operated is determined by (1) how quicklythe APD can be quenched once a photon has been detected and (2) howquickly the APD can be re-armed once it has been quenched.

Although quenching stops the avalanche process, not all free carriersare instantaneously swept out of the avalanche region. Instead, somecarriers become trapped in the multiplication region in trap energystates, which arise from crystalline defects or other causes. Thesetrapped carriers are released in a temporally random manner based onsuch factors as temperature, the type of trap state, and the appliedbias voltage. When a trapped carrier is released after the SPAD hasalready been re-armed, there is a possibility that it can initiateimpact ionization as if the APD has absorbed a photon. As a result, thedetrapping of a carrier can result in a “false” electrical signal thatoccurs in the absence of photon absorption. A false count that occurs inthe absence of a photon absorption is referred to as a “dark count,” anddark counts that arise specifically from detrapping of trapped carriersare referred to as “afterpulses.”

The temporal variation in the rate of dark counts constitutes noise in asingle-photon avalanche detector. As a result, afterpulses degrade SPADsensitivity. One approach for improving sensitivity in the presence ofafterpulsing is to simply delay rearming after quenching. This allowstrapped charges a sufficient period of time to detrap while the SPADremains unarmed. Unfortunately, such an approach requires an undesirablylong period of time when the single-photon detector is insensitive toincident photons.

Alternative approaches for reducing afterpulse effects include 1)actively inducing rapid detrapping of trapped charges; 2) stifling thedetrapping of trapped charges; and 3) limiting the number of freecarriers that flow through the multiplication region during an avalancheevent.

Actively induced detrapping can be accomplished in several differentways, such as heating the photodiode or energizing the carriers byilluminating them with light at a different wavelength. Such approaches,however, have shown very limited success. Elevating the temperature ofan APD imposes a severe tradeoff by increasing the dark count rate whilesub-bandgap illumination has not yet been shown to effectively inducecarrier detrapping. In addition, these approaches increase cost andcomplexity, making these approaches undesirable in many applications.

The stifling of trapped charges by lowering the temperature of a SPAD to“freeze” trapped charge carriers has not been successfully demonstrated.In fact, for practical SPAD devices, this approach is likely to increaseafter-pulsing as temperature is reduced. Further, if carrier freeze-outwere successful, it is likely that at least some of the charge carriersassociated with the dopant atoms would also be “frozen,” thus renderingthe SPAD inoperable.

Some afterpulse reduction has been successfully demonstrated through theuse of external circuitry to limit the flow of free carriers through themultiplication region during an avalanche event. However, thecapacitance associated with the external circuitry adds to the RC timeconstant that dictates the rate at which a SPAD can be rearmed afterquenching. For high-speed operation (i.e., >1 MHz), this RC timeconstant must be less than about 1 microsecond. As a result, anycapacitance associated with external electronics that adds to thecapacitance of the SPAD itself is generally undesirable. Moreover, theuse of external circuitry generally involves significant complexity, andit can lead to additional undesirable parasitic elements in addition tothe capacitance just described.

Monolithic, passive quenching approaches for limiting the flow of freecarriers by have also been explored. The use of a monolithicallyintegrated quenching (or feedback) load element can help to avoid theexcessive parasitic capacitance inherent in the use of externalcircuitry. However, this legacy “negative feedback avalanche diode”(NFAD) is limited by an inherent tradeoff: a large feedback load isdesired to promote rapid quenching, but a small feedback load isnecessary to enable rapid re-arming. In prior-art NFADs, a singlefeedback load value is chosen to balance this tradeoff between thetimescales for quenching and re-arming and, therefore, overall deviceperformance is necessarily compromised.

SUMMARY OF THE INVENTION

The present invention enables high-speed operation of single-photondetectors without some of the costs and limitations of the prior art.Embodiments of the present invention include an NFAD that comprises aSPAD coupled with a dual-state feedback system. The feedback systemcomprises a feedback element that has a first state during quenching anda second state during re-arming, wherein the first state has a largeload and the second state has a small load. The state of the feedbackelement is controlled by an electrical signal that originates within theSPAD itself. Embodiments of the present invention are particularly wellsuited for use in single-photon detection applications requiringoperation at rates greater than 10 MHz.

In operation, the feedback system enables strong quenching (i.e.,provides a “large” load via the feedback element) during the SPADavalanche until the quench is complete and then rapidly changes thestate of the feedback element to enable fast recharging (i.e., providesa “small” load via the feedback element) to re-arm the device. As soonas re-arming is complete, the feedback element again assumes “large”load characteristics for the next avalanche to be quenched.

An illustrative embodiment of the present invention comprises a feedbacksystem that includes a feedback element and a second “control” element,which is able to induce the change in the feedback element between largeload and small load. The control element is operates via an electricalsignal provided by the SPAD itself.

In some embodiments, the feedback element functions as a switch with avery high impedance state for quenching and low impedance state forrecharging. In some embodiments, the feedback element comprises atransistor with appropriate ON/OFF state properties.

In some embodiments, the control element provides a temporal delayappropriate for delayed switching of the load between its two states.

An embodiment of the present invention comprises: an avalanchephotodiode dimensioned and arranged to enable single-photon detection; aload element having a first load state and a second load state, thefirst load state being higher impedance than the second load state; anda control element; wherein the load element and control elementcollectively define a load for the avalanche photodiode, and wherein theload element switches between the first load state and second load statebased on a control signal generated within the photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of an NFAD in accordance with theprior art.

FIG. 2 depicts a schematic drawing of an NFAD in accordance with anillustrative embodiment of the present invention.

FIG. 3 depicts operations of a method for operating NFAD 200 inhigh-frequency Geiger mode in accordance with the illustrativeembodiment of the present invention.

DETAILED DESCRIPTION

The following terms are defined for use in this Specification, includingthe appended claims:

-   -   Single-photon avalanche diode (SPAD) is defined as an avalanche        photodiode structure designed and intended for operation in        Geiger mode.    -   Negative feedback avalanche diode (NFAD) is defined as a SPAD        that is operatively coupled with a negative feedback load.        Preferably, the negative feedback load and SPAD are        monolithically integrated.    -   Electrically connected is defined as being in direct electrical        contact. Two terminals are considered electrically connected if        each remains at the same voltage potential for substantially any        magnitude of electrical current through them (neglecting any        inadvertent voltage drop through a conductive electrical line or        trace used to connect the terminals together).

FIG. 1 depicts a schematic drawing of an NFAD in accordance with theprior art. NFAD 100 comprises SPAD 102 and load 104.

SPAD 102 is an avalanche photodiode that is dimensioned and arranged toprovide a measurable output current in response to the receipt of photon106, which has a wavelength within the wavelength range for which SPAD102 is operable.

Load 104 is a conventional discrete resistor having a value within therange of approximately 10 KΩ approximately 10 MΩ. Load 104 iselectrically connected with the anode of SPAD 102 to form a seriescombination of elements. In some cases, load 104 is a different load,such as one or more resistive and/or reactive linear elements,non-linear devices such as a transistor, and various combinations ofthese devices.

In operation, Vbias is applied across the series combination of SPAD 102and load 104. Vbias is a voltage above the breakdown voltage, Vbr, ofSPAD 102, as a result, Vbias “arms” the SPAD, which enables the SPAD togive rise to an electric current in response to the receipt of a singlephoton.

Prior to incidence of a photon on SPAD 102, no current flows through theseries combination of SPAD 102 and load 104; therefore, the voltage dropacross load 104, V_(L), is substantially zero. Upon receipt of a photon,however, SPAD 102 avalanches giving rise to a macroscopic current flowthrough load 104 and voltage drop V_(L) to increase. As a result, thevoltage drop, V_(D), across SPAD 102 reduces to a value very close tothe breakdown voltage, thereby causing the avalanche to quench.

A principal shortcoming of the NFAD shown in FIG. 1 is that a load withsufficient impact to rapidly quench a SPAD avalanche typically requiresa long recharge time for re-arming SPAD 102. To illustrate, considerthat for an effective purely resistive load, a larger resistance wouldprovide a more rapid quench. Unfortunately, it also would require alonger time for recharging the SPAD due to its associated large RC timeconstant, which is dictated by the product of the load resistance andthe inherent capacitance of the SPAD 102.

FIG. 2 depicts a schematic drawing of an NFAD in accordance with anillustrative embodiment of the present invention. NFAD 200 comprisesSPAD 102, load 202, and sense resistor R_(S).

Sense resistor R_(S) is a conventional resistor having resistance ofapproximately 50Ω and is typically integrated with SPAD 102 in hybridfashion. Although in the illustrative embodiment sense resistor R_(S)has a resistance of approximately 50Ω, it will be clear to one skilledin the art, after reading this Specification, how to specify, make, anduse alternative embodiments of the present wherein sense resistor R_(S)has a resistance that is other than 50Ω.

Load 202 includes load element 204 and control element 206.Collectively, load element 204 and control element 206 define adual-state load for SPAD 102, wherein the impedance of the load is basedon a control input that is obtained from the output of the SPAD itself.In the illustrative embodiment, load 202 and SPAD 102 are monolithicallyintegrated; however, in some embodiments, load 202 and SPAD 102 aredifferent elements that are integrated in another manner that yields ahighly integrated system. Integration methods suitable for use with thepresent invention include, without limitation, flip-chip bonding,plasma-assisted bonding, wafer bonding, die bonding, and the like.

FIG. 3 depicts operations of a method for operating NFAD 200 inhigh-frequency Geiger mode in accordance with the illustrativeembodiment of the present invention. Method 300 begins with operation301, wherein NFAD 200 is armed by providing V_(bias) between terminals208 and 218 (i.e., across the series combination of sense resistorR_(S), SPAD 102, and load 204), where V_(bias) is greater than thebreakdown voltage, Vbr, of SPAD 102. In the absence of an incidentphoton, substantially no current flows through SPAD 102; therefore,there is an insignificant voltage drop across each of load 202 and senseresistor R_(S); therefore, almost all of V_(bias) develops across SPAD102. In other words, prior to the receipt of a photon, V_(D) issubstantially equal to V_(bias), and develops between terminals 210 and212.

At operation 302, SPAD 102 absorbs photon 106. Photon 106 is a singlephoton having a wavelength greater than 1000 nm. Because SPAD 102 isbiased above its breakdown voltage, receipt of this single photonresults in an avalanche event that gives rise to macroscopic currentI_(D), which flows through the series combination of SPAD 102 and load202. Although in the illustrative embodiment, SPAD 102 is operable todetect a photon having a wavelength that is greater than 1000 nm, itwill be clear to one skilled in the art, after reading thisSpecification, how to specify, make, and use a SPAD that is operable fordetecting photons having wavelengths other than just those greater than1000 nm.

At operation 303, output signal 220 is provided at terminal 210. Outputsignal 220 is the voltage drop that arises from the flow of I_(D)through sense resistor R_(S).

At terminal 212, current I_(D) is divided into current I_(L), whichflows through load element 204, and current I_(C), which flows throughcontrol element 206.

Load element 204 is a field-effect transistor that has two distinctimpedance states between terminals 212 and 218, which are electricallyconnected with the source and drain of the transistor. The impedancestate of load element 204 depends upon control signal 214, which isprovided to load element 204 at terminal 216, which is electricallyconnected to the gate of the transistor. In the absence of a suitablecontrol input at terminal 216, load element 204 has a very highimpedance state between terminals 212 and 218. When a suitable controlsignal is applied to terminal 216, however, load element 204 has a lowimpedance state between terminals 212 and 218. In some embodiments, loadelement 204 is an element, other than a field-effect transistor, havingan impedance that can be alternated between high impedance and lowimpedance based on the application of a control signal. Load elementssuitable for use with the present invention include, without limitation,CMOS transistors, NMOS transistors, bipolar-junction transistors, andthe like.

At the onset of operation 303, the magnitude of control signal 214 issubstantially zero. As a result, there is no voltage applied to the gateof the transistor and load element 204 is in a high impedance state. Theflow of current I_(L) through load element 204, therefore, quickly givesrise to a large voltage drop, V_(L).

At operation 304, voltage drop V_(L) reduces the magnitude of thevoltage drop across SPAD 102 to a magnitude sufficiently close to itsbreakdown voltage, Vbr, to quench the avalanche event caused by thereceipt of photon 106. Preferably, quenching of NFAD 200 occurs withinapproximately 1 ns from the receipt of photon 106.

At operation 305, current I_(C) flows through control element 206.

Control element 206 is a resistive circuit element having a resistancesuitable for quickly developing a suitable voltage drop betweenterminals 212 and 216 in response to the flow of current I_(C), whichresults in an increase in the magnitude of control signal 214 sufficientto induce load element 204 to toggle into its low impedance state.

Although in the illustrative embodiment control element 206 comprises aresistive circuit element, it will be clear to one skilled in the art,after reading this Specification, how to specify, make, and usealternative embodiments of the present invention wherein control element206 comprises a reactive circuit element or an active circuit element.Reactive circuit elements suitable for use in control element 206include, without limitation, capacitors, inductors, and the like. Activecircuit elements suitable for use in control element 206 include,without limitation, CMOS transistors, NMOS transistors, bipolartransistors, and the like.

At operation 306, SPAD 102 charges at a rate determined by the chargingtime constant (i.e., RC time constant) of SPAD 200. Once the charge onSPAD 102 results in V_(D) exceeding Vbr, NFAD 200 is re-armed and readyto detect the arrival of another photon. One skilled in the art willrecognize that the RC time constant of NFAD 200 is substantially definedby the capacitance, C, of SPAD 102 and the resistance, R, of load 202.When load element 204 is in its low-impedance state, therefore, NFAD 200is quickly re-armed. Preferably, NFAD 200 re-arms within approximately 1ns.

It is an aspect of the present invention that control signal 214 isgenerated internally to NFAD 200. By way of contrast, prior-artapproaches for controlling the load on a SPAD have relied upon the useof externally generated signals for control, which requires complicatedcircuitry and complex control algorithms, or a single-state passivequenching element that requires a tradeoff between quenching time andre-arming time. As a result, the present invention enables highfrequency SPAD operation with potentially lower cost and/or complexityrelative to the use of external circuits, as well as higher performanceoperation relative to the use of single-state passive quench elements.

It should be noted that capacitance at terminal 216 (i.e., the gatecapacitance) of load element 204 and the resistance of control element206 also collectively define an RC time constant that acts as a delay tocircumvent the simultaneity problems inherent in some non-linear singleelement configurations. The delay introduced by the control element 206is sufficiently long to ensure that the load remains in its “quenchingstate” (i.e., high impedance) for a sufficient period of time toeffectively quench an induced avalanche event before switching the loadto its “recharging state” (i.e., low impedance). Ideally, this delay isas short as possible to avoid excess avalanche charge flow, which wouldlead to undesirably large afterpulsing in the NFAD. Preferably, thetotal time required to quench NFAD 200 is less than about 1 ns. In fact,if possible, this time is much less than 1 ns to enable even higheroperation rates.

In similar fashion, the combination of load element 204 and controlelement 206 provides a second (i.e., re-arming) delay that ensures thatthe “recharging state” is maintained long enough for the recharging tobe completed before the load is again switched back to its “quenchingstate”. This re-arming delay is also preferably 1 ns or less. In someembodiments, however, although a longer re-arming delay compromisesdetector availability, it can be tolerated in order to allow more timefor charge detrapping and, thus, improve afterpulsing in the NFAD. Forsome applications, embodiments of the present invention have re-armingdelays within the range of approximately 1 ns to approximately 100 ns.Further, in some other applications, counting rates as low as 1 MHz canbe tolerated as long as the NFAD provides extremely low afterpulsing(e.g., due to a very long effective hold-off time). In some embodimentsof the present invention, therefore, the re-arming delay of NFAD 200 canbe as long as 1 microsecond.

An embodiment of the present invention includes a negative feedbackavalanche detector comprising: a single-photon avalanche diode; loadelement having a first load state and a second load state, the firstload state being higher impedance than the second load state; and acontrol element operable for providing a control signal that determinesthe load state of the load element; wherein the load element and controlelement collectively define a load for the single-photon avalanchediode, and wherein the load element switches between the first loadstate and second load state based on a control signal generated withinthe negative feedback avalanche detector.

It is to be understood that the disclosure teaches just one example ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

What is claimed is:
 1. A negative feedback avalanche detectorcomprising: a single-photon avalanche diode; a load element having afirst load state and a second load state, the first load state beinghigher impedance than the second load state; and a control elementoperable for providing a control signal that determines the load stateof the load element; wherein the load element and control elementcollectively define a load for the single-photon avalanche diode, andwherein the load element switches between the first load state andsecond load state based on a control signal generated within thenegative feedback avalanche detector.
 2. The negative feedback avalanchedetector of claim 1 wherein the control signal is based on a signalprovided by the single-photon avalanche photodiode.
 3. The negativefeedback avalanche detector of claim 2 wherein the control signal isbased on a first current through the single-photon avalanche photodiode.4. The negative feedback avalanche detector of claim 1 wherein thecontrol element comprises a resistor, and wherein the control signal isbased on a first current from the single-photon avalanche photodiode,the first current being a portion of the avalanche current, and whereinat least a portion of the first current passes through the resistor. 5.The negative feedback avalanche detector of claim 1 wherein the loadelement comprises a transistor.
 6. The negative feedback avalanchedetector of claim 1 wherein the single-photon avalanche photodiode, theload element, and the control element are monolithically integrated. 7.The negative feedback avalanche detector of claim 1 wherein thesingle-photon avalanche photodiode is dimensioned and arranged to detecta single photon having a wavelength equal to or greater than 1000 nm. 8.The negative feedback avalanche detector of claim 1 wherein the controlelement and the load element collectively define a delay element.
 9. Thenegative feedback avalanche detector of claim 1 wherein the controlelement comprises at least one element selected from the groupconsisting of a resistor, a reactive element, and a transistor.
 10. Amethod comprising: generating a first current in response the receipt ofa first photon at a negative feedback avalanche photodiode; providing afirst portion of the first current to a load element, wherein the loadelement has a first load state and a second load state, and wherein thefirst load state is higher impedance that the second load state, andfurther wherein the load state of the load element is based on a firstcontrol signal; providing a second portion of the first current to acontrol element; providing the first control signal from the controlelement, the first control signal being based on the second portion. 11.The method of claim 10 further comprising providing the control elementand the load element such that the load element switches from the firstload state to the second load state in response to an increase in themagnitude of the first current and the load element switches from thesecond load state to the first load state in response to a decrease inthe magnitude of the first current.
 12. The method of claim 11 whereinthe control element and load element are provided such that (1) the loadelement switches from the first load state to the second load stateafter a first time delay after the increase in the magnitude of thefirst current and (2) the load element switches from the second loadstate to the first load state occurs after a second time delay after thedecrease in the magnitude of the first current.
 13. The method of claim10 further comprising providing the load element such that the loadelement includes a transistor.
 14. The method of claim 10 furthercomprising providing the control element such that it includes aresistor.
 15. The method of claim 10 further comprising providing thecontrol element such that it includes a reactive element.
 16. The methodof claim 10 further comprising providing the control element such thatit includes an active element.